1) Field of the Invention
The present invention relates to an image processing apparatus, such as a copying machine, a printer, a facsimile, that detects an edge of an image, and carries out an optimum processing to an edge portion.
2) Description of the Related Art
Conventionally, a digital color image processing apparatus like a digital color copying machine first reads, with a scanner, R (red), G (green), and B (blue) signals of a draft as reflectance data. The digital color image processing apparatus then carries out a color processing that includes a conversion processing for converting the reflectance data into a concentration value, and color correction/ink generation/under-color removal (UCR) processing. In this process, the digital color image processing apparatus converts the data into image data corresponding to four color recording materials including C (cyan), M (magenta), Y (yellow), and K (black), and carries out a pseudo intermediate tone processing, and a gamma processing. Finally, the digital color image processing apparatus outputs a reproduced image from a printer.
However, when there are both the characters and the pictures, it is necessary to carry out different appropriate processing to the characters and to pictures. There is know an apparatus that identifies different characters at various positions of an image, and controls the processing by using identification signals.
As the identification processing, there is an image area separation processing. The image area separation processing is a processing of making reference to an image in a relatively large area, and detecting a black character edge, a color character edge, and a picture area. The digital color image processing apparatus controls the processing based on a result of the image area separation processing. In general, a result of an image area separation processing is produced as binary information of 0 and 1. Therefore, a defect could easily occur due to a rapid changeover of a processing. Problems of the conventional processing will be explained taking the ink generation and UCR processing as examples. In the ink generation processing, a K signal is generated from C, M and Y signals. In the under-color removal processing, a quantity corresponding to the K signal (under-color removal quantity K′) is subtracted from the C, M and Y signals.
In FIG. 1 shows a case in which a black character edge of a relatively low concentration ([1]) is an input image. A K single color is reproduced for a portion ([2]) that is decided as a black character edge in the image area separation processing. An inside portion of a character that is not a black character edge is reproduced in CMY (K and CMY become signals of [3] and [4] respectively) . The purpose of reproducing the character edge portion in a single K color is to prevent the coloring around the character. The reason for reproducing CMY is because the use of the K signal increases the granular feeling which is not desirable in the picture highlight, although the same ink generation processing is carried out for the inside portion of the character and the picture portion. However, when the K signal is color deviated to the left and this is printed by the printer as shown in [5], for example, a gap is generated at the boundary, and a “crack” occurs. A phenomenon that white of the paper appears in the gap spoils the view, and this is a large defect. Therefore, the image area separation processing has a basic problem of a defect when the separation is in error.
An apparatus that carries out identification, other than the image area separation processing, has been disclosed in Japanese Patent Application Publication No. H7-108019, for example. This apparatus controls the under-color removal quantity at multi-stages according to the edge and the black component. When the black generation and the UCR are controlled at multi-stages by expressing the characteristics in multi-values, there are the following merits and demerits.
FIG. 2 shows an example in which a gray low concentration character edge is used as an input image ([1]) The control is at multi-stages using multi-value characteristics ([2]) . K and CMY change smoothly as shown in [3] and [4] respectively. Therefore, a crack that occurs in the control based on binary information shown in FIG. 1 does not occur unless an extreme color deviation occurs. However, when the K signal deviates to the right as shown in [5], a “coloring” occurs at the outside of the character edge. When there is a large color deviation at the time of inputting an image from the scanner or when the MTF characteristic is poor, a remarkable coloring occurs particularly in a thin line.
There is also know an apparatus that uses an adaptive processing of both the image area separation of binary control and the multi-stage control (for example, an apparatus as described in Japanese Patent Application Laid-Open No. 10-173916). From the viewpoint of the defect due to an erroneous separation, this apparatus carries out an adaptive processing to low-point fine polygonal characters that cannot be easily decided by the separation. Based on this, the image quality is corrected.
However, the adaptive processing of the above apparatus functions only as an assistant to character edges for which the image area separation processing does not work. This apparatus does not solve the problem of the “crack” that occurs at the character edge portions for which the image area separation processing works.